Hearing aid microphone



H. B. SHAPIR O HEARING AID MICROPHONE 9 1 Z 1 w A Filed Aug. 3, 1940 INVENTOR ATTORNEY Patented Aug. 17 i943 UNITED STATES PATENT OFFICE HEARING AID MICROPHONE 7 Harry B. Shapiro, Astoria,.Long- Island, N. Y., 25-

signor to Sonotone Corporation, Elmsi'ord,

N. Y., a corporation or New York ,p

Application August 3, 1940, Serial No. 350,595

7 Claims. This invention relates to hearing aid microphones, and particularly to diaphragm-driven hearing, aid microphones having vibro-electric transducer elements which are sensitive to variations in humidity, such as Rochelle saltfcrystal microphones or electret-type condenser microphones.

Among the objects of the invention is a microphone of the foregoing type in which the driving its operation extremely light and small so that the complete electron tube haring aid may be comfortably and inconspicuously worn on the body of the user.

The electron tube amplifier of such hearing aid must be very small and flat so as to fit into a srophone of the foregoing type which is supported by a yieldable diaphragm behind a soundpervious casing wall portion, so as to hold the microphone spaced from the casing walls and form an acousticseal around the space between the sound-pervious casing wall portion and the facing microphone diaphragm; the recessing ofthe air-pervious microphone casing wall portion relatively to the adjacent casing boundary which is exposed to contact with the users garment; and the arrangement of the yieldable microphone support relatively to masses associated with the microphone casing so that they prevent and suppress mechanical propagation of noise vibrations from the surface of the microphone casing to the yieldably suspended microphone unit.

The foregoing and other objects of the invention will be best understood from the followin description of exemplifications' thereof,'-.'-reference being had to the accompanying drawin'g wherein 1 Fig. l is a vertical cross-sectional view'along line |I of Fig. 2 of a hearing aid microphone exemplifying the invention; Fig. zis a view along line 2--2 of Fig.' 1.;

Fig. 3 Ba vertical sectional view along line 3-3 of Fig. 4 illustrating a wearable electron tube amplifier unit with the microphone of Figs. 1 and 2 mounted therein;

Fig' 4 is a front elevational view of the microphone of Fig. 3 with parts broken away;

Fig. 5 is a view similar to Fig. 1 illustrating a modified arrangement oi the microphone; and

Fig. 6 is a front view of the microphone of 'the microphone for driving the amplifier,

High impedance microphones, such as Rochelle salt crystal or electret-type condenser microphones, have characteristics which make them desirable for-use in such electron tube hearing aid amplifiers. However, prior to the invention,

' the structure of such microphones was yery bulky to which their active elements are exposed.

Thus, for instance, the output of a Rochelle salt crystal or an electret microphone drops to almost zero when, under exposure to humidity, its surface leakage reduces its impedance to a fraction of the required operative value.

The requirements with respect to high sensitivity and good frequency response, while protecting the humidity-sensitive transducer structure against variations of humidity in the surrounding air, imposed very strict limitations on the minimum size.of such hearing aid microphone, since because of the high sensitivityrequ'ired from a hearing aid microphone, it must be driven by a relatively large diaphragm ex-' posed tothe sound propagated in the surrounding air.

If an attempt is made to house such diaphragm unit in a very fiat casing that is sealed against humidity, the stifiness of the ,air in the small casing cavity greatly reduces its sensitivity in the frequency range below 1000 cycles and makes it practically useless as a hearing aid microphone.

The effect of the stifiness of the air in reducing the sensitivity of such small completely sealed microphone is overcome by providing the casing wall with an acoustic vent which is sealed with a humidity-proof diaphragm that has such ing conically shaped light vibratory diaphragm it having its peripheral edge sealed'to the flange ii." of a flat shallow casing wall it; so as to form a very fiat cavity. The diaphragm i i is very thin so as to reduce its mass, and its outwardly tapered shallow conical shape, in combination with the peripheral corrugation it, gives it the desired stifiness, while confining its dimensions to a very thin space in its roomiest central part which is just deep enough to house a Rochelle salt crystal transducer unit it. As an alterna tive, an electret-type condenser transducer, structure may be mounted in the cavity and ar ranged to be actuated by the diaphragm.

In the form shown, a standard Rochelle salt crystal bimorph it of the twister type is used having overall dimensions of "2?; x 7 x of an inch. Three corners of the crystal unit are secured across spacer pads ii to the inner face of the casing wall it, the spacer pads ll being of a slightly yieldable highly viscous material. To the corner of the crystal it which is located underneath the vertex of the conical diaphragm ii is secured a metallic mounting strip it or a driving link iii-J, the outer end of which is united,

as by cementing, to the tip of the diaphragm ii so that vibrations of the diaphragm are trans= mitted to the himorph i6.

One electrode layer of the crystal it is connected through a foil strip 2i to a terminal tip 22 which is riveted over an insulating strip 2 3 to the rear wall it of the casing, the insulating strip 26 serving also as an insulating support for another terminal tip 23 to which is connected a similar foil strip 251 extending from the other electrode layer or layers of the crystal. Two insulated cord leads 25 connected to the two terminal tips 32,23 and tightly sealed by vapor-proof, seals in small holes of the cavity wall it provide the external microphone output leads. The junction between the diaphragm ii and the casing i3 and all other junctions and entrances into the microphone cavity are sealed bystrong cementitious material which is impervious to water or other vapors and forms a good cementitious junction. Bakelite 6052 cement or Vulcalock give satisfactory results. microphone cavity sealed against humidity with such cements maintains the sensitivity of the Rochelle salt crystal or electret type transstructure-is reduced to provide just enough space for housing the transducer structure.

As a result, the thin layer of air confined in the cavity has high stiffness which cuts down the sensitivity 2,327, rec

of the microphone in the range down below about 1000 cycles so that such microphone would be of no practical value for a hearing aid microphone.

In the microphone of the invention, the sensitivity in the low frequency range is raised by forming in the flat casing wall it vent holes 36 having an acoustic inertance which, in combination with the stifiness in the air of the microphone cavity, forms a vibrating system which is resonant in the desired part of the lov. frequency range, for instance, around 409- or 505 cycles, thereby raising the sensitivity of the microphone in the range between 500 and i008 cycles so that notwithstanding the small ness of the microphone structure cavity and the great stifiness or the air layer confined therein, it is as efiective as any prior bulky hearing aid microphone of a similar type.

The interior of the microphone structure cavity is maintained sealed against humidity notwithstanding the provisions of the vent holes 3 i, by providing around the portion of the cavity wall it; in which the vent holes 3i are formed a peripheral ridgeii'i? to which is united, as by cementing, a sheet element of thin flexible material 33 so as to form a vapor-proof seal thereon.

In the actual construction of such microphone, a thin flexible piece of aluminum toll is used as the sealing layer 33, its stifiness mass being kept so small compared to the stiffness oi the air in the cavity of the microphone struct re and the acoustic inertance of the vent holes, to assure that the vent holes 3i have the full efiectiveness that they would have in the absence of the sealing layer It was found that such sealing layer is effective in sealing the cavity against humidity, while leaving fully efiective the action of the vent holes 3i, if the resonance of the yieldable layer is made of the order of the resonance of the system formed by the acoustic vents 3i and the stlfiness or" the air in the cavity. A sealing layer having a resonance at about 400 cycles and vent holes 3i which, in combination with the stiffness of the'air in the microphone cavity, form a system resonant at 500 cycles, resulted in a satisfactory, extremely compact hearing aid microphone, with a microphone structure having a thickness of only .165 to .110 inch. The resonance peaks of the vibratory system so formed may be suitably damped, as by cementing over the vent holes iii a piece of sound-pervious vibration damping fabric 84!.

A microphone using a Rochelle salt crystal transducer unit and designed in the way de-' scribed above ha'san overall sensitivity and performance of the same character as the best similar diaphragm-driven hearing aid microphones heretofore available, although occupying only a small fraction of the space required by such prior microphones.

A high impedance'microphone used as a part of a high gain electron amplifier hearing aidhas to operate under conditions utterly different than those encountered in other uses. This is due to the fact that the amplifier casing in which v the microphone is usually housed has to be worn ,more or less hidden under the clothing of the userand connectedby a cordto the receiver, resultmg in noise vibrations imparted to the cord terior of the casing to the microphone.

According to the invention, the. effect of cloth rubbing noises on the microphone output are The supporting collar practically suppressed by'providing the amplifier casing with an acoustically-pervious recessed wall portion behind which the microphone is mounted, so as to formin front of the diaphragm an air space which is acoustically sealed against the back space of the amplifier casing.

In addition, the microphone unit is so mounted behind the sound-pervious wall of the amplifier casing that it is freely supported on a yieldable spacer diaphragm having a great vibration absorbing capacity and impeding propagation of noise vibrations from the casing wall to the microphone.

One form of such arrangement is shown in ,connection with Figs. 3 and 4 illustrating the fiat compact casing small enough for inconspicuous wear in a vest pocket of the user in which is housed an electron tube amplifier and a microphone arranged in the way described in my copending application SerialNo. 294,649, filed September 13, 1939, the specification of which is and damping "sheet material, such as Neoprene or suitable rubber, has an outer peripheral portion folded and united, as by cementing, over a supporting collar 44 to fprm in front thereof'a shaped ridge formation 41' against which the sealing head 45 is pressed into sealing engagement. As shown in Fig. 3, the supporting collar 44 is held in its proper position by three brackets 48,having legs resting against the elements of the electron tube amplifier assembly mounted,

within the casing behind the microphone unit.

By recessing the acoustically-pervious wall portion 4| of the casing behind which the microphone is mounted relatively to the other wall f surfaces which form the outer boundary of the casing 40, garment parts which come in contact with the outer boundary of the casing are kept away from theedges of the vent openings 42 and generation of noise by rubbing thereon is prevented, and by keeping the microphone yieldably suspended and spaced from the amplifier casing unit, mechanical propagation of noise to the microphone is suppressed.

In Figs. 5 and 6 is shown a modified form of microphone arrangement of the invention. In

. this arrangement, the'delicate diaphragm M of the microphone is protected'by a dome-shaped circular plate 5| having openings 52 through which sound is propagated to the diaphragm ll of the microphone; The periphery of the microphone is yieldably supported by the intermediate portion of a collar 53 of yieldable material, such as rubber, having a forwardly extending lip 54 held in engagement with the facing inner border surface of the recessed wall portion it of the amplifier casing so as to form an acoustic seal interposed between the space in the back of the microphone and the space between'the diaphragm ll sealing bead 45 forming a seal around the soundpervious casing wall It against which it is pressed, and a radially spaced inner portion 46 of the supporting diaphragm 43 is united, as by cementing, to the periphery of the microphone unit so as to hold it fioatingly spaced from the casing wall and all other structural elements housed in the casing 4E9, while forming an acoustic seal around the space between the microphone diaphragm ii and the recessed wall portion 4|.

As explained in. my copending application Serial No. 294,649, the electron tube amplifier of I the invention utilizes a magnetic core inductance and a magnetic core transformer as a part of the amplifier circuit, the inductance and the core having a substantial mass and being mechanically joined to the casing 48;

By interposing the relatively large mass of the amplifier casing, which is increased by the mass of the magnetic cores of the coupling inductance and the transformer, between thev yieldable vibration absorbing diaphragm 43 which holds the microphone unit floatingly spaced from the cas ing structure, the mechanical, propagation of against contact with'the casing and any other elements housed therein, while keeping it yieldably suspended on the yieldable annular diaphra'gm 43 in the, inner space of the ring, and at the same .time providing an acoustic seal for the and the recessed acoustically-pervious wall, the rear edge 55 of'the yieldable collar engaging and resting on the underlying portions of the electron tube amplifier assembly which occupies the underlying part of the microphone casing.

The rear wall 56 of the extremely thin microphonecavity has a very shallow rearwardly depressed wall portion 51 in which is formed a vent hole 58 which is sealed by a light thin diaphragm 59 cemented to the inner edges of the depressed wall portion, so as to give a vapor-tight humidity seal for the flat microphone cavity, while at the same time keeping efiective the vent action of the hole 58, as explained in connection with Figs. 1 and 2. r V

Theprinciples of the invention explained in connection with specific exemplifications thereof will suggest to those skilled in the art many other applications and modifications of the same. It is accordingly desired that the appended claims be construed broadly, and that they shall not be limited to the specific details shown and described in connection with exemplifications thereof.

1 ranged to produce in response to sound acting space between the diaphragm and the acoustically-pervious recessed wall 4| of the casing.

The portion of the recessed casing wall 41 facing the supporting collar 44 has an annularly on said diaphragm a corresponding, electric outputfsaid transducer means being of such characteristics that its output is substantially reduced when it is subjected to predetermined variations in the composition of the atmosphere in which it operates; saidcavity enclosurehaving dimensions at which the stiffness of the air confined therein would, in the'absence of cavity vents, reduce sublow i000 cycles; and a light flexible diaphragm forming a substantially vapor-proof seal between theair within said cavity enclosure and said vent cycles; and a light flexible diaphragm forming a substantially atmosphere-proof seal between the air within said cavity enclosure andsaid vent opening portions and having its rriasses and stifi-- ness so arranged and proportioned as to substantially maintain the effectiveness of the acoustic inertance oi said vent opening portions with respect to the stifiness of the air confined in said.

cavity. a Y

2. In a microphone unit for a hearing aid that is small enough for inconspicuous wear on the body of the user: a generally flat relatively rigid member forming a cavity wall; a vibratory diaphragm having a periphery joined to said wall to form therewith a shallow vapor-tight'cavity enclosure; vibro-electric transducer means including an element enclosed in said cavity and connected to be driven by said diaphragm so as to produce in response to sound acting on said diaphragm a corresponding electric output; said transducer means being of such characteristics that its output is substantially reduced when it is subjected to predetermined variations in the compositionof the atmosphere in which it operates; said cavity enclosure having dimensions at which the stiffness of the air confined therein would, in the absence of cavity vents, reduce substantially the sensitivity of the action of said diaphragmin the frequency range of about 1000 cycles and below; said cavity wall having vent opening portions to the air space on the exterior side of said cavity wall exhibiting an acoustic inertance which resonates with the air of said cavity enclosure at a frequency in the range below 1000 cycles; and a light flexible diaphragm phragm having a periphery joined to said wall opening portions and having its masses and stifiness so arranged and-proportioned as to substantially maintain the efiectiveness of the acoustic inertance of said vent opening portions with respect to the stiffness of the air confined in said cavity.

4. In a microphone unit for a hearing aid that is small enough for inconspicuous wear on the body of the user: a generally flat relatively rigid member forming a cavity wall; a vibratory diato form therewith a shallow vapor-tight cavity enclosure; transducer means including a vibroelectric transducing element enclosed in said cavity and connected to be driven by said diaphragmso as to produce in response to sound acting on said diaphragm a corresponding electric output; said transducing element being of such characteristicsthat its eihciency is substantially reduced when it is operated under predetermined excess humidity conditions in the atmosphere; said cavity enclosure having dimensiohs at which the stiffness of the air confined proof seal between the air within said cavity enclosure and said vent opening portions and havforming a substantially atmosphere-proof seal between the air within said cavity enclosure and said vent opening portions and having its masses and stiffness so arranged and proportioned as to substantially maintain'the effectiveness of the acoustic inertance of said vent opening portions with respect to the stii fne'ssof the air confined in said cavity.

3. In a microphone unit for a hearing. aid that is small enough for inconspicuous wear on the body of the user: a generally flat relatively rigid member forming a cavity wall; a vibratory dia: phragm having a periphery joined to said wall to form therewith a shallow vapor-tight cavity enclosure; transducer means including a vibroelectric transducing element enclosed in said cavity and arranged to produce in response to sound acting on said'diaphragm a corresponding electric output; said transducing element being of such characteristics that its efficiency is sub ta tially reduced when it, is operated under predetermined excesshumidity conditions in the atmosphere; said cavity enclosure having dimensions at which the stifiness of the air confined therein would, in the absence of cavity vents, re-

duce substantially the sensitivity of the action cit said diaphragm in the frequency range of about 1000 cycles and below; said cavity wall havin vent opening portions to the air space on the exterior side of said cavity wall exhibiting an acoustic inertance which resonates with the air of said cavity enclosure at a frequency in therange being its masses and stiffness 50 arranged and proportioned as'to substantially maintain the effectiveness of the acoustic inertance of said vent opening portions with respect to the stiffness of the air confined in said cavity.

5. In a microphone unit for a hearing aid that is small enough for inconspicuous wear on the body of the user: a generally fiat relatively rigid member forming a cavity wallf a vibratory diaphragm having a periphery joined to said wall to form therewith a shallow vapor-tight cavity enclosure; transducer means including a vibroelectric transducing element enclosed in said cavity and connected to be driven by said diaphragmso as to produce in response to sound acting on said diaphragm a corresponding electric output; said transducing element being of such characteristics that its efficiency is substantially reduced when it is operated under predetermined excess humidity conditions in the atmosphere; said cavity enclosure having dimensions at which the stiffness of the air confined therein would, in the absence of cavity vents, reduce substantially the sensitivity of the action of said diaphragm in a part of the frequency range of about 1000 cycles and below; said cavity wall having vent opening portions to the air space on the exterior side of said cavity wall exhibiting an acoustic inertancewhich resonates with the air of said cavity enclosure at a frequency in the range below 800 cycles; and a light flexible diaphragm forming asubstantially vapor-proof seal between the air within said cavity enclosure and said vent opening portions and 7 having its masses and stifiness so arranged and vent opening portions with respect to the stiflness of the air confined in said cavity.

6. In a wearableelectron-tube amplifier hearing aid, a fiat amplifier casing small enough for substantially hidden wear on the body of the comprising a vibratory diaphragm located adjacent said sound pervious -wall portion so as to be actuated by sound propagated therethrough; a generally fiat relatively rigid microphone cavity wall member having a periphery joined to said diaphragm to form therewith a shallow vapor-tight microphone cavity enclosure of a thickness of the order of one-half the thickness of said casing or less; transducer means including a vibro-electric transducing element enclosed in said cavity and arranged to produce in response to sound acting on said diaphragm a corresponding electric output; said transducing element being of such characteristics that its efficiency is substantially reduced when it is operated under predetermined excess humidity conditions in the atmosphere; said microphone cavity enclosure having dimensions at which the stiffness of the air confined therein would, in the absence of cavity vents, reduce substantially the sensitivity of the action of said diaphragm in a part of'the frequency range of about 1000 cycles and below; said cavity wall having vent opening portions to the air space within said casing on the exterior side of said cavity wall and being arranged sa that said vent-opening portions exhibit an acoustic inertance which resonates with the air of said cavity at a frequency in the range below 1000 cycles; anda light flexible diaphragm forming a substantially vapor-proof sealbetween the air within said cavity and said vent opening portions and having its masses and stiflnes's so arranged and proportioned as to substantially maintain the effective-.

nessoi the acoustic lnertance of said vent opening portions with respect to the stiffness of the air confined in said cavity.

7. In a wearable electron-tube amplifier hearing aid, a fiat amplifier casing small enough for substantially hidden wear on the body of the user and having two walls enclosing the opposite sides of a relatively shallow interior space; one of said walls having a sound pervious wall portion; a

, microphone unit located in said interior casing space for cooperation with the amplifier elements of said hearing aid; said microphone comprising a vibratory diaphragm located adjacent said sound pervious wall portion so as to be actuated by sound propagated therethrough; a

generally flat relatively rigid microphone cavity wall member having a periphery joined to said diaphragm to form therewith a shallow vaportight microphone cavity enclosure of a thickness of the order of one-half the thickness of said casing or less; transducer means including a vibro-electric transducingelement enclosed in said cavity and connected to .be driven by said diaphragm so as to produce in response to sound acting on said diaphragm a corresponding electric output; said transducing velement being of I such characteristics that its efiioiency is substan tially reduced when it is operated under predefrequency range of about 1000 cycles and below; 1

said cavity wall having Vent'opening portions to the air space within said casing on the exteriorside of saidcavity wall and being arranged so that said vent opening portions exhibit an acoustic inertance which resonates. with the air of said cavity at a frequency in the range below 1000 cycles; and a light flexible diaphragm forming a substantially vapor-proof seal between the air within said cavity and said vent opening portions and having its masses and stillness so arranged and proportioned as tov substantially maintain the effectiveness of the acoustic inertance of said vent opening portions with respect to the stiffness of the air confined in said cavity.

HARRY B. SHAPIRO. 

